For conventional fluid-flow electrical-generation turbine systems, such as wind turbine systems, in which the energy source is variable (i.e. the fluid speed and the rate of flow of the fluid varies over time), the amount of energy captured from the energy source may only be a fraction of the total of the energy that may be capturable over time. For example, in a typical wind farm, that fraction may be one half, or less.
The power flow though a variable-speed conventional turbine/generator/transformer system is restricted in the range of power it can output, i.e., from a minimum output power to a rated output power, because of limitations of the generator, the power converter (if present), and the output transformer used within the system. This restriction arises because a conventional electromagnetic generator has reduced efficiency at lower power levels, as does the power converter (if present) and particularly the transformer that couples power to the electrical load. As a result, for the conventional variable-speed turbine/generator/transformer system an engineering design decision is usually made to limit the power rating of the generator (and any associated power converter, power conditioner or power filter, if present) and the associated output transformer so as to optimize efficiency over a restricted range of power. Therefore, at the extremes of normal-operating fluid speeds, i.e., at a low fluid speed and especially at a high fluid speed, less power is coupled into the turbine than it is possible to extract from the fluid energy source. For a given design of turbine diameter (and possibly axial length) this translates, over time, into less energy capture than the turbine may be capable of transmitting to the generator.
To increase energy capture in situations in which the energy source has a variable speed of fluid driving the turbine, and in which the turbine may have a variable speed of rotation, a multi-stage generator may be used in the turbine system. A multi-stage generator is an electromagnetic machine operating as an electrical generator that takes mechanical energy from a prime mover and generates electrical energy, usually in the form of AC power. Such a multi-stage generator is disclosed in U.S. Pat. No. 7,081,696 and U.S. Patent Application Publication No. 2008/0088200, which are both hereby incorporated by reference. An advantage of a multi-stage generator over a conventional generator is that a multi-stage generator can be dynamically sized depending on the power output of the turbine. A conventional generator is effective at capturing energy from the energy source over a limited range of fluid speeds, whereas a multi-stage generator is able to capture energy over an extended range of fluid speeds of the energy source, due to staged power characteristics.
The electrical power that is generated from a multi-stage generator is variable in nature, meaning the output power waveforms produced may vary from time to time, for example in: voltage amplitude; current amplitude; phase; and/or frequency. Additionally a multi-stage generator may include a number of induction elements, each of which generates its own power waveform, which may differ in voltage amplitude, current amplitude, phase, and/or frequency, from that generated by other induction elements within the generator. An electrical load such as an electric utility power grid may not be capable of consuming directly the electrical power that is generated by a multi-stage generator, as the power generated may not be in the correct form, for example, with respect to waveform shape as a function of time, voltage amplitude, current amplitude, phase, and/or frequency, as may be required by the electrical load. An electrical load such as a utility power grid typically expects from a turbine electrical generation system a single-phase, or split-phase, or 3-phase voltage or current waveform that is usually sinusoidal, and relatively stable, but a multi-stage generator generates varying waveforms.
A power converter circuit may be used to transform electrical power waveforms from one form to another form. Converters may be designed for a specific rating of the input voltage range (e.g. 1000 VAC-rms to 2000 VAC-rms) and input current range rating (e.g. 100 A-rms to 500 A-rms), but if the input voltage or input current (and therefore power level) do not meet or exceed the levels for which the converter is designed, then the converter may not be capable of operation, or the converter may operate in an inefficient manner. For a multi-stage generator a single power converter is unlikely to accommodate the widely varying voltage waveforms and power range that is generated. Moreover, a single power transformer delivering power to the electrical load, connected to one or more converters, is unlikely to accommodate with reasonable efficiency the wide range of power that may be generated by a multi-stage generator.